Low temperature firing multilayer ceramic circuit boards are known that are suitable for use with low melt temperature conductive metals, such as silver, gold and copper. They have a low thermal coefficient of expansion (TCE) and they may be formulated to be compatible with both silicon and gallium arsenide devices.
These ceramic circuit boards are made from glasses that can be fired at low temperatures, e.g., temperatures of less than 1000° C. The circuit boards are made by admixing finely divided selected glass particles or powders and optional inorganic fillers, with organic materials including resin, solvents, dispersants and the like. The resultant slurry is cast as a thin tape, called green tape. A circuit pattern may be screen printed onto the green tape using a conductor ink formulation comprising a conductive metal powder, an organic vehicle and a powdered glass, generally the same glass as that used to make the green tape.
A plurality of green tapes having printed circuits thereon can be stacked together. In such case, via holes are punched into the green tapes which are filled with a conductive via fill ink to provide electrical contact between the circuits on the various green tapes. The green tapes are then aligned, laminated under heat and pressure, and fired to remove the organic materials and to vitrify the glass.
Recently, multilayer ceramic circuit boards have been adhered to a metal support board for added mechanical strength. A bonding glass can be used to coat the metal support and to provide adhesion between the support and the laminated ceramic layers. An added advantage to this method is that the bonding glass reduces shrinkage of the green tapes in the x and y dimensions during firing. Thus most of the shrinkage occurs in the z, or thickness, dimension. The result is that tolerances between the circuits and the via holes can be reduced. The glasses used to make the green tapes must have a TCE matched to that of the metal support to prevent delamination or cracking of the fired glass. The TCE of the green tapes can be modified by use of various metal oxide glass precursors and various inorganic fillers.
Still more recently, various passive components, such as resistors and capacitors, have been incorporated into this ceramic circuit board system. Discrete components initially were mounted on the fired green tape stack, and wire bonded to circuitry placed about the edges of the circuit board. Presently components such as resistors and capacitors are being printed on green tape layers where they become embedded in and part of the circuit board after firing.
Such systems can be used with RF and microwave components, particularly in the fields of personal communication, wherein manufacturers wish to produce devices, among them hand held devices, that are small, light in weight, more reliable and less expensive than conventional devices. One of the critical components of such systems are the provision of RF filters which are required to define and separate RF frequency bands at radio and microwave frequencies with minimum loss and maximum selectivity. Presently such RF filters are made as discrete, surface mounted components, e.g., edge-coupled stripline resonators, which are expensive. Further, they take up valuable board space that could be given over to incorporation of additional functions on the board, or to reduce the overall size and weight of the ceramic circuit board.
Embedded RF filters including strip conductors in a ceramic circuit board stack have been tried, but the performance results are no more than marginal for insertion loss and selectivity.
Thus a method of forming and embedding RF filters in a green tape stack, that can be fired without loss of performance, has been sought.